1. Field of Invention
This invention relates to a structure for the low loss connection of an optical fiber to an integrated optical waveguide having different dimensions from the fiber. More specifically it relates to a mode shape transformer fabricated as an integrated optical waveguide structure that is used for changing the shape and dimension of an optical mode field of an integrated optical waveguide so that it matches the mode field shape of an input optical fiber.
2. Background of the Invention
The invention relates to the field of semiconductor and dielectric optical components used for optical transmission or optical digital data processing. It pertains especially to all optical components comprising active and/or passive waveguides and also to single and multimode fibers, for example, components such as semiconductor lasers, semiconductor amplifiers, modulators, and wavelength filters, to name a few. There is a great deal of ongoing research and development effort to reduce the cost and size of optical modules while maintaining the minimum loss of optical power. A major component of the fabrication costs usually arises from the joining of such components to optical fibers, as for example when it is necessary to couple optical modes of very different sizes. Thus, when a high-index contrast (HC) waveguide and a flat-end single-mode optical fiber are joined together, the optical mode of the HC guide spot size with a diameter of, for example, 1-2 μm, has to be coupled with the optical mode of a single mode (SM) optical fiber whose diameter is far greater, for example, in the range of 6-10 μm.
To enable the coupling of these optical modes of very different sizes, spot-size converters or mode shape transformers (or mode transformer for short) are made in order to increase the size of the mode at the output of the optical component and make its profile compatible with that of the mode guided in the optical fiber. The reverse can also be accomplished to decrease the size of the mode from a single mode (SM) fiber to a high-index contrast (HC) waveguide. In any case, this mode matching must be done while preserving the performance characteristics of the component.
For instance, in directly connecting a SM fiber having a mode spot size of 8 μm, to a HC waveguide having a spot size of 1.5 μm, over 90% of the power will be lost. Such loss is intolerable in optical communication systems. FIG. 1 shows the relative sizes of the modes (field patterns) of a conventional SM fiber (100), and that of an HC waveguide. (110). The SM fiber spot size is typically 5-10 μm which is as much as an order of magnitude greater than that of an HC waveguide—typically 1-2 μm.
When transforming the modes between two waveguides with different refractive index and/or core sizes, high coupling loss arises due to the difference in mode size, shape, and mode velocity. For example, the index difference and the mode size of a fiber optic waveguide are different than those of a high index difference planar waveguide, resulting in high coupling loss when the fiber optic waveguide and the high index difference planar waveguide are directly coupled.
A channel waveguide is a dielectric waveguide whose core is surrounded by a cladding that is comprised of materials with refractive indices lower than that of the core, and wherein the peak optical intensity resides in the core. Waveguides in general can be defined by other geometries as well. In all waveguide cases, the cladding is defined as a region where the evanescent field of optical modes exists. A high index contrast (HC) waveguide is defined as one where the core cladding index contrast is larger than that of a typical single mode fiber (that is, larger than approximately 1%). HC waveguides also typically have mode field diameters that are smaller than that of a single mode fiber by a factor of two or more.
In optical components, it is essential to have low coupling loss when attaching a fiber to a microchip. To efficiently couple two waveguides with very different dimensions and therefore two different spot sizes, some sort of mode shape transformer is required. As shown in FIG. 2, the mode shape transformer essentially acts as a funnel, necking down a wide area spot from the SM fiber (200) to a small area spot associated with the HC waveguide (220). FIG. 2 depicts the functionality of such a mode transformer (210)
A mode transformer between two different waveguides is an essential part of an optical system where the lightwave (mode) from one optical component is coupled into another component. In optical communication, a mode transformer between an optical fiber waveguide and a high index difference (difference in the refractive indices of core and cladding) planar waveguide is crucial for successful implementation of planar lightwave circuits (PLC) in fiber communication. Therefore, developing an efficient mode transformer between two waveguides has continued to be a subject of intense development.
In addition, the core index of the fiber optic waveguide is lower than that of the high index difference planar waveguide causing a mode velocity difference between two waveguides. As will be detailed in the Detailed Description section, when such a change in mode properties takes place too quickly, high power loss arises.
3. Prior Art
There have been several other approaches to achieve efficient mode coupling between two waveguides with different index difference, including mode transformation by tapering the dimension of high index difference waveguides. Mode transformation by a taper has been shown in various publications. Over the tapering region of the high index difference waveguide, the thickness or width of the waveguide core is gradually tapered down from that of the normal guiding region to a lower thickness or width. As the mode travels from the normal guiding region of the high index difference waveguide into the tapering region, the mode experiences decreasing amount of the core material. The fraction of the mode field distribution that exists outside the core material increases, changing the mode size. The index of the waveguide that the mode experiences is effectively changed by the presence of the taper. In other words, the “effective index” is gradually changed by the taper. By gradually changing the effective index from that of the low index waveguide to that of the high index difference waveguide, the mode coupling can be achieved between two waveguides without high loss. The method to determine the effective index is described in “The Handbook of Photonics”, Boca Raton, Fla., CRC Press, 532-535 (1996) by M. Gupta.
T. Brenner et. al. (“Integrated optical modeshape adapters in InGaAsP/InP for efficient fiber-to-waveguide coupling,” IEEE Photonics Tech. Lett. Vol. 5, No. 9, 1993) shows a mode transformer using a vertically tapered high contrast waveguide. Vertical tapering uses special etching techniques that are not well-controlled and therefore difficult to manufacture. Also the vertical taper shape cannot be arbitrarily specified but is more a function of etching characteristics, rather than design. The mode size propagating in the tapered region increases due to the reduction of the effective index, and thus the reduction of the effective index difference. The publication indicates the gradual mode transformation occurring in one waveguide due to the presence of a taper.
U.S. Pat. No. 5,199,092, issued to Stegmueller et al., shows the coupling of modes between two different waveguides: one broad and one narrow. The two waveguides run parallel to one another and are superimposed with each other to provide superimposing waveguide guidance. During the superimposed waveguide guidance, one of the two waveguides is tapered down in vertical dimension, while the other waveguide dimension is kept constant. The role of the tapered waveguide is to provide a gradual effective index change, and thus mode transformation, in the same manner as the cases cited in journal publications including that by Brenner et al. The difference is the superimposition of the narrow waveguide, providing waveguiding in the broad waveguide once the narrow waveguide is completely terminated by the vertical taper. The broad waveguide is surrounding the narrow waveguide over the whole waveguiding distance. The presence of the broad waveguide helps guiding the mode once the mode transformation is complete.
In addition to single taper devices described above, dual tapers are used in mode transformation between two different waveguides. IEEE Photonic Technology Letters, Vol. 7, No. 5, May 1995 by Zengerle et al., reports a mode transformer having two channel waveguides, each with a taper, one sitting on top of the other. Electronics Letters, Vol. 29, No. 4, February 1993 by Schwander et al., reports a mode transformer having two rib waveguides, each with a taper, a portion of one embedded within the other. Both of the rib waveguides used in the art are weakly guiding. This is not a suitable method for mode transformation to or from a high index difference waveguide.
Y. Shani et. al. (“Efficient coupling of a semiconductor laser to an optical fiber by means of a tapered waveguide on silicon”, Appl. Phys. Lett. vol. 55, No. 23, 1989) describes a mode transformer using a taper embedded within a second larger waveguide. Their taper is however adiabatic. In that case, the taper was wedge shaped (linearly tapered) and very long in order to make use of the adiabatic mechanism. The taper is required to also come down to a sharp point, which makes it almost impossible to perform in a lithographic process.
B. M. A. Rahman et. al. (“Improved laser-fiber coupling by using spot-size transformer”, IEEE Photonics Technology Lett. Vol. 8, No. 4, 1996) describes a mode transformer using two synchronously coupled waveguides, where one guide is a smaller, high contrast guide and the other has a spot size approximating the size of a fiber mode. Their mode transformer does not use any mode evolution process, and the coupling is not terminated, causing coupling to periodically transfer between the two guides indefinitely.
G. A. Vawter et. al. (“Tapered rib adiabatic following fiber couplers in etched GaAs materials for monolithic spot-size transformation,” IEEE J. Selected Topics Quantum Electronics, Vol. 3, No. 6, 1997) shows an adiabatic coupler from one waveguide to another where the high contrast waveguide is on top of the larger fiber-matched waveguide.
Variations of the above types of mode transformers can also be found in several review articles, including that by I. Moerman et. al. (A review of fabrication technologies for the monolithic integration of tapers with III-V semiconductor devices,” IEEE J. of Selected Topics Quantum Electronics,” Vo. 3, No. 6, 1997) which summarizes primarily dual type waveguide tapering.
In U.S. Pat. No. 6,253,009 entitled “SEMICONDUCTOR OPTICAL COMPONENT COMPRISING A SPOT-SIZE CONVERTER”, the invention relates more particularly to a semiconductor optical component, comprising an active waveguide and a passive waveguide that are superimposed and buried in a sheathing layer, wherein the component comprises successively: a damped coupling section in which the width of the active waveguide tapers down and the width of the passive waveguide increases, and a mode expansion section comprising only the passive waveguide whose width tapers down. According to another characteristic of the invention, the component furthermore comprises a transition section positioned between the damped coupling section and the mode expansion section, in which the width of the active waveguide tapers down to 0 μm. The invention enables the making of an optical component comprising an integrated spot-size converter, wherein the optical mode is chiefly deconfined in the passive guide so much so that the current threshold and the efficiency of the component are not affected. The passive and active waveguides are not aligned but they are coupled vertically, so much so that the problems related to alignment are prevented. Furthermore, the two types of active and passive waveguide may be optimized separately.
A third approach that has been envisaged in order to obtain a spot-sized converter integrated into an optical component requires an active waveguide and a passive waveguide that are superimposed so as to create a damped vertical coupling zone, wherein the width of the active waveguide tapers down gradually to deconfine the mode, and the width of the passive waveguide increases very rapidly and then becomes constant throughout the length of this section. The two guides, namely the active guide and the passive guide, are furthermore buried in a sheathing layer. In this case, the width of the coupling section must be sufficient to enable complete deconfinement of the optical mode in the entire active guide. This length is generally greater than 150 μm. Furthermore, as and when it gets deconfined, the optical mode transits through the passive guide. The passive guide has a constant width of about 4 μm and a very small thickness of about 50 nm to enable the deconfinement of the mode. For, an excessive thickness of this passive guide would prevent the deconfinement of the mode in the active guide. This is why the passive generally has a thickness of less than 100 nm.
The major drawback of this approach lies in the fact that the deconfinement of the mode is done entirely in the active guide. This leads to a deterioration of the performance characteristics of the component, especially its threshold current and its efficiency. Furthermore, the optical coupling losses with a single-mode optical fiber are still high. They are about 4.5 dB.
None of the prior art provides for an efficient mode transformation between a low index difference and a high index difference waveguide on a microchip. This invention discloses, for the first time, an efficient optical mode transformer based on a taper design, useful for transforming the mode to a high index difference waveguide on a semi-conductor microchip. The matching of the optical mode according to the invention is performed chiefly in the HC waveguide that is embedded on the semiconductor microchip.